The main story is probably around 8-12 hours. Then there is at least 15-20+ hours of side quests. Beyond that, if you're a collector, it will probably take quite a long time to find all of the feathers, chests, and pages.
Just the campaign seems a bit shorter to me than some of the previous games. But there is so many side missions that are actually fun to do that it makes the game seem longer. I'm also of the opinion that the naval missions are the most fun part of the game, and they are pretty much all optional.